Sheet of autogenously bonded polytetrafluoroethylene fibers and method of producing same



United States Patent 3,186,897 SHEET 0F AUTOGENOUSLY BONDED POLY- TETRA'FLUOROETHYLENE FIBERS AND METHOD OF PRODUCING SAME Jerome Hochberg,.Newbnrgh, N.Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Nix-Drawing. Filed July 17, 1962, Ser. No. 210,557 6. Claims. (Cl. 161 -150) This invention relates to a process for making nonwoven-sheetmaterials and more particularly to a process for making non-woven sheet materials of polytetrafluoroethylene.

Because of its outstanding properties such as heat and chemical resistance, polytetrafiuoroethylene has found wide use in'recent years. Non-woven, felt-like sheet materials of fibrous polytetrafluoroethylene are used in applicationssuch as, for example, wicks for corrosive liquids, gaskets, electrical and heat insulation, chemically resistant clothing. and filters.

Non-woven sheet materials of polytetrafluoroethylene are generally prepared by the process which comprises forming a non-woven batt of the fibers, needle punching the resulting batt and heat-shrinking the resulting product to form a stable non-woven sheet material. Polytetrafluoroethylene is a very slippery material and, because of this, non-woven batts of polytetrafluoroethylene are extremely difficult to handle before and while they are needle punched. Such'batts pull apart and stretch easily. Therefore, it has been very difficult to form thin non-woven sheets of polytetrafluoroethylene. Furthermore, needle punching non-woven batts of polytetrafiuoroethylene, although an excellent method for compacting the batts and adding strength thereto, is anadditional step requiring relatively expensive machinery. Also, non-woven sheet materials of polytetr'afiuoroethylene prepared by the aforementioned general procedure tend to ravel and have a surface fuzz of fibers projecting from the surface of the products; Non-woven sheets of polytetrafiuoroethylene in which the sheets are strengthened and the fibers are bound with polymeric binders or fused fibers have the disadvantage that the material which binds the non-woven fibers limits the heat resistance and, in many cases, the chemical resistance thereof.

I have found a method for making non-woven sheets of fibrous polytetrafluoroethylene which does not require the use of a binder, does not require a needle punching step and can be used to'mak'e very thin sheets of polytetrafiuoroethylene and sheets substantially free from raveling and surface fuzzin'g.

The process of this invention comprises (a) forming a non-woven batt of fibers consisting essentially of polytetrafluoroethylene, (b) pressing the batt at a temperature be low about 550 F., and preferably 200 to 400 F. at a pressure of about from 100 to 2,000, and preferably 400 to 800 pounds per square inch, and (c) heating the resulting product without pressing at a temperature between the fusing temperature of the fibers and about 700 F., and preferably about from 630 to 650 F.

Any of a wide variety of fibrous forms of polytetrafiuoroethylene, both drawn and undrawn but drawable, can be used in the process of this invention. These include, for example, staple and mono-filament fibers of polytetrafiuoroethylene and yarns thereof. Fibers ranging from 0.5 to 16 denier, or even 100 denier or higher can be used. Staple fibers ranging from 2 to 10, and preferably 4 to 6 inches in length and having a denier of 0.5 to 16, and preferabiy 2 to 8 are particularly preferred. US. Patent No. 2,559,750, for example, describes a process for preparing fibers useful in this invention which comprises extruding an aqeuous dispersion of polytetrafiuoroethylene to form 3,186,897 Patented June 1, 1965 filaments which are then'sintered to make the fibers drawable. The'term fibers-consisting essentially of polytetrafluoroethylene as used herein is intended to cover fibers of homopolyrners of tetrafiuoroethylene as well as copolymers of tetrafiuoroethylene with minor amounts, usually- 15% or less,- ofhexafiuoropropene, monochlorotrifiuoroethylene, vinylidene fluoride, vinyl fiuoride and the like which have essentially the chemical. andphysical properties of homopolymers of tetrafluoroethylene.

The first step in the process of this invention comprises forming a non-Woven batt of fibers. This can be done by any of the. conventional means such as, for example, carding and cross-lapping the fibers, depositing thefibers on a moving belt, conveyor, drum or the like with air or forwarding jets or by collecting the fibers on perforated screens from dispersions in'water, air or other fluid media.

Next, the non-wovenbatt of fibersconsisting essentially of polytetrafluoroethyleneis pressed. This is done at a temperature substantially below the fusing temperature (also known as transition temperature) of the fibers at any temperature between about room temperature and about 550 F. The pressing can'be done in conventional equipment such as, for example, presses, calenders or the like. The pressed batt offibers consisting essentially of polytetrafiuoroethyleneis then heated at a temperature above the fusing temperature of the fibers (usually about 621 F.) to about 700 F. without pressing the fibers. The duration of this heat treating step varies'with the temperatureat whichthe fibers areheated. Generally, the pressed, non-woven batts are heated for about from 10 to minutes. Preferably the battsv are heated for about from 45 to 15- minutes at about from 630 to 650 F. At the higher temperatures approaching 700 F., the period of heating is shortened to preserve the fibrous form of the polytetrafluoroethylene.

It has been surprisingly found that, if the non-Woven batts. are treated v as. described above, the fibers consisting essentially of polytetrafiuoroethylene, unlike fibers of other materials, maintain their. fibrous nature even though the fibers are held at a temperature above their fusing point. This treatment causes the fibers in the prepressed mat to adhere at their fiber intersections but does not destroy the fibrous nature thereof. The products thus produced comprise non-woven sheet materials of discrete fibers consisting essentially of polytetrafiuoroethylene substantially all of- Which lie substantially parallel to the plane of the sheet materials.

By the process of this invention it is. possible to make non-woven sheets of fibers consisting essentially of polytetrafluoroethylene which do not contain a binder, which are not needle punched and which are substantially free from surface fuzzing and raveling. The process of this invention is particularly suited for making very thin nonwoven batts of polytetrafiuoroethylene. For example, single slivers of polytetrafluoroethylene can be formed on sheet metal or other suitable supports, pressed, then heated without pressing as described hereinbefore to yield light, porous products-weighing 011 the order of an ounce or even less per square yard. The products of this invention are useful, for example, for heat and chemical resistant filters, carpets and underliners, heat and sound insulation, wicks, gaskets, protective clothing and protective coverings for ironing boards, presses and the like and as substrates for impregnated sheet materials.

The following examples are intended to illustrate the invention and not to limit it in any way. Parts and percentages are by weight unless otherwise indicated.

Example 1 Drawn, 6.7 denier, 4.5-inch polytetrafiuoroethylene staple fibers are carded and cross-lapped into a non-woven batt about 40 mils thick consisting of about from 10 to 15 layers of fibers or slivers cross-lapped at an angle of about 90. The resulting batt is pressed at a temperature of about 392 F. under a pressure of 650 pounds per square inch for about 15 minutes, then transferred to a cold press and given an additional pressing at a pressure of 650 pounds per square inch. The pressed batt is placed in an oven and heated without pressing for 30 minutes at 650 F., then removed and cooled.

The product of the example is substantially free from surface fuzzing and raveling. The product also has the following properties:

Average weight, oz./ sq. yd 38 Average thickness, mils 52 Tensile strength, lb./in. 5O Elongation at break, percent 100 Gurley porosity, sec/100 cc. of air at 20 oz./ sq. in.

pressure 0.56

The Gurley porosity is determined on the apparatus and by a procedure similar to that described in Federal Specification CCCT191b, Method 5452.

Examples 2 through 5 Temperature of Average Average weight, Examples heat treatment, thickness, oz./sq. yd.

F. mils Also, each of the products described above is substantially free from surface fuzzing and raveling and contains discrete unfused fibers.

Example 6 A non-woven batt of 4.5 inch, 6.7-denier polytetrafluoroethylene fibers weighing about 0.3 ounce per square yard is pressed for 1.5 minutes at a temperature of 392 F. and a pressure of 800 pounds per square inch, then heated without pressing for 30 minutes at 650 F. The resulting product has an average thickness of about 7.5 mils and weighs about 1.6 ounces per square yard. This product is useful for a high capacity air filter.

Example 7 Two slivers of 6.7-denier, 4.5-inch polytetrafiuoroethylene staple fiber are cross-lapped at an angle of about 90 to yield a batt weighing about 1.0 ounce per square yard. The resulting batt is pressed at 392 F. at a pressure of 900 pounds per square inch for 1.5 minutes, then heated without pressing for 30 minutes at 650 F. The resulting product has an average thickness of about 13 mils and weighs about 5.6 ounces per square yard.

Example 8 Continuous filament polytetrafluoroethylene yarn r EARL M. BERGERT, Primary Examiner.

formed from 180 two-denier monofilaments per thread is deposited randomly on a moving conveyor with forwarding jets. The resulting batt weighs about 40 ounces per square yard. The non-woven batt is then pressed at a temperature of about 250 F. at a pressure of about 800 pounds per square inch, then heated without pressing for 45 minutes at 630 F. to yield a non-woven sheet material useful, for example, as electrical and heat insulation.

This application is a continuation-in-part of my parent application, Serial No. 780,681, and now abandoned.

I claim:

1. A process which comprises (a) forming a non-woven batt of fibers consisting essentially of polytetrafiuoroethylene, (b) pressing the batt at a temperature of less than about 550 F. and a pressure of about from 100 to 2,000 pounds per square inch and (c) releasing said pressure and heating the resulting product at a temperature between about the fusing temperature of the fibers and about 700 F. to produce a non-Woven sheet of discrete fibers lying substantially parallel to the plane of the sheet and being adhered together by fusion at fiber intersections.

2. A process which comprises (a) forming a non-Woven batt of fibers consisting essentially of polytetrafluoroethyl ene, (b) pressing the batt at a tempertaure of about 200 to 400 F. at a pressure of about from 400 to 800 pounds per square inch and (c) releasing said pressure and heating the resulting product at a temperature of about from 630 to 0 F. for about from 45 to 15 minutes to produce a non-woven sheet of discrete fibers lying substantially parallel to the plane of the sheet and being adhered together by fusion at fiber intersections.

3. A non-woven sheet of discrete fibers consisting essentially of polytetratluoroethylene, substantially all of said fibers lying substantially parallel to the plane of said sheet and being adhered together by fusion at fiber intersections.

4. The process of claim 2 wherein the fibers are staple fibers. V

5. The process of claim 2 wherein the fibers are continuous filaments.

6. A process which comprises (a) forming a non-woven batt of fibers consisting essentially of polytetrafluoroethylene, (b) pressing the batt at a temperature substantially below the fusion temperature, and (c) heating the resultant product without pressing at a temperature above the fusion temperature and for a time sufiicient to produce a non-woven sheet of discrete fibers lying substantially parallel to the plane of the sheet and being adhered together by fusion at fiber intersections.

References Cited by the Examiner UNITED STATES PATENTS 2,374,540 4/45 Hall 2646 2,539,329 1/51 Sanders 117126 2,728,698 12/55 Rudner 16l93 2,764,506 9/56 Piccard 117-119.2 2,773,781 12/56 Rodman 161-92 3,003,912 10/61 Harford 162157 FOREEGN PATENTS 796,610 6/ 58 Great Britain. 

3. A NON-WOVEN SHEET OF DISCRETE FIBERS CONSISTING ESSENTIALLY OF POLYTETRAFLUOROETHYLENE, SUBSTANTIALLY ALL OF SAID FIBERS LYING SUBSTANTIALLY PARALLEL TO THE PLANE OF SAID SHEET AND BEING ADHERED TOGETHER BY FUSION AT FIBER INTERSECTIONS. 